Setup Steps for Copilot Coding Agent in Phoenix Apps

Coding Agent README.md

Setup Steps

Workflow File

Add copilot-setup-steps.yml to your .github/workflows directory so Copilot Coding Agent doesn’t get lost trying to set up Elixir and Erlang.

This also ensures a postgres database is running, which is helpful for Phoenix apps out of the box.

Dependencies are cached to make the agent start up quickly, as well as saving on metered GitHub Actions compute time.

Copilot Instructions

Add copilot-instructions.md to your .github directory to provide instructions to the agent.

These instructions provide a simple baseline, establishing the expectation to format code and describing how to run tests / do basic troubleshooting.

copilot-instructions.md

Running Tests

To run the test suite:

mix test

Troubleshooting

Database Connection Issues

If you encounter database connection issues, ensure:

• The database user has the correct credentials (username: postgres, password: postgres)
• The database exists (created by mix ecto.create or mix setup)

Asset Build Issues

If assets fail to build, try:

mix assets.setup
mix assets.build

Formatting

Make sure all of the code is properly formatted according to Elixir styles.

You can format the code using:

mix format --check-formatted

copilot-setup-steps.yml

name: Copilot Setup Steps

on: workflow_dispatch

jobs:
  copilot-setup-steps:
    runs-on: ubuntu-latest
    permissions:
      contents: read

    # Set up a Postgres DB service. By default, Phoenix applications
    # use Postgres. This creates a database for running tests.
    # Additional services can be defined here if required.
    services:
      db:
        image: postgres:17
        ports: ["5432:5432"]
        env:
          POSTGRES_PASSWORD: postgres
        options: >-
          --health-cmd pg_isready
          --health-interval 10s
          --health-timeout 5s
          --health-retries 5

    steps:
      # Step: Setup Elixir + Erlang image as the base.
      - name: Set up Elixir
        uses: erlef/setup-beam@v1
        with:
          otp-version: "27.3.4"
          elixir-version: "1.18.4"

      # Step: Check out the code.
      - name: Checkout code
        uses: actions/checkout@v4

      # Step: Define how to cache deps. Restores existing cache if present.
      - name: Cache deps
        id: cache-deps
        uses: actions/cache@v3
        env:
          cache-name: cache-elixir-deps
        with:
          path: deps
          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
          restore-keys: |
            ${{ runner.os }}-mix-${{ env.cache-name }}-

      # Step: Define how to cache the `_build` directory. After the first run,
      # this speeds up tests runs a lot. This includes not re-compiling our
      # project's downloaded deps every run.
      - name: Cache compiled build
        id: cache-build
        uses: actions/cache@v3
        env:
          cache-name: cache-compiled-build
        with:
          path: _build
          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
          restore-keys: |
            ${{ runner.os }}-mix-${{ env.cache-name }}-
            ${{ runner.os }}-mix-

      # Step: Download project dependencies. If unchanged, uses
      # the cached version.
      - name: Install dependencies
        run: mix deps.get